The present invention relates to radiographic imaging systems using flat panel imaging arrays, and in particular, to such systems having automatic exposure control (AEC) sensing capability.
Referring to FIG. 1, AEC is used in radiography to control the x-ray dosage delivered to the receptor. Typically, a sensor, separate from the image acquisition devise itself, is positioned in front of or behind the receptor, and senses the x-ray exposure in real time. The AEC device provides an output signal 1 in real time, usually in the form of an analog voltage that is proportional to the total delivered x-ray exposure. This signal 1 is used by x-ray source to terminate the exposure when the signal 1 identifies the exposure as having reached a predefined threshold 2.
Ideally, it would be desirable to use the receptor itself to sense the exposure and provide the AEC signal in real time. However, using the actual image acquisition device itself is problematic for the reason that it is intended to capture the signal on a frame-by-frame basis, and not in real time. For example, in flat panel digital radiography, the exposure is integrated on each pixel and then read out, typically, every few seconds. The x-ray beam duration is defined in tens of milliseconds, so real time in this context would require millisecond or sub-millisecond updates to the total AEC signal value.
Referring to FIG. 2, in addition to this problem of time scaling, traditional AEC detectors for radiography provide an AEC signal that is proportional to specific regions of the total image, e.g., typical AEC sensing regions 3a, 3b, 3c used for chest radiography.
Referring to FIG. 3, in a conventional flat panel digital radiography system, an x-ray source 4 irradiates a patient 5 with an x-ray beam 6. The radiation 6a not blocked or absorbed by the patient 5 is received by the flat panel detector 7. Typically, an external AEC detector (not shown) associated with the flat panel detector 7 provides the AEC signal 8, which is monitored by a controller 9 to provide an appropriate x-ray control signal 10 to terminate x-ray emissions when an exposure level sufficient to create a diagnostically useful image has been achieved. While external AEC detectors generally work well, they add significant cost to the overall system. Further, at least two external AEC detectors are often required since, unlike many flat panel detectors, they are not intended to move between table and chest stands, but are built into the x-ray table or chest stand.